Cochlear implants are used to provide a sensation of hearing to hearing impaired persons. Typically, the implant provides stimuli via a set of electrodes formed into an array which is inserted into the scala tympani of the patient. The cochlear implant system presents electrical stimulation directly to the auditory nerve fibres of the basilar membrane. The electrodes are driven via an implanted receiver stimulator unit. The implanted receiver stimulator unit produces stimulations in accordance with commands originating from an external speech processor. A preferably transcutaneous link transfers power and commands from the speech processor unit to the receiver stimulator.
The inner ear of a normally hearing person includes hair cells which convert the displacement of the basilar membrane in response to sound into nervous impulses. Different parts of the basilar membrane of the normal cochlea are displaced maximally by different frequencies of sound so that low frequency sounds maximally displace apical portions whereas higher frequency sounds cause displacement of more basal portions of the membrane. The nervous system is arranged so that a nervous impulse originating from a hair cell located adjacent an apical area of the membrane is perceived as a low frequency sound whereas a nervous impulse originating from a hair cell located adjacent a more basal position of the membrane is perceived as a higher frequency sound. The frequency which causes maximal displacement of the basilar membrane at a given position will hereinafter be referred to as the "characteristic frequency" at that position.
In a dysfunctional ear the hair cells may be damaged or absent so that no nervous impulses are generated. In such a case electrical stimulation impulses must be provided artificially to simulate the nervous activity of the hair cells in order to create a perception of sound. Such stimulation impulses are provided via the electrodes of a multi-channel cochlear electrode array. The array is arranged to follow at least part of the length of the basilar membrane and its electrodes are selectively driven to deliver electrical stimulations. In order to simulate a given sound it is necessary to firstly analyse that sound and break it down into essential features. This analysis can be in accordance with many different schemes and is performed by the speech processor. The speech processor then determines which electrodes of the array should be stimulated in order to best simulate the sound. For example, if the sound contains mainly high frequency components then it is best simulated by stimulation via basally located electrodes.
In order to determine the electrode to be stimulated for a given sound the speech processor makes use of a frequency range to electrode map, usually stored in an EPROM, which matches bands of sound frequencies to one or more electrodes of the electrode array. The frequency range mapped to each electrode is adjustable by the speech processor so that a characteristic frequency is allocated for each stimulating electrode. Existing methods for allocating frequency ranges to the electrodes are to use an educated guess or a longhand calculation to determine the characteristic frequency for each electrode and to choose frequency ranges consistent with the characteristic frequencies calculated for the electrodes.
It is accordingly desirable to be able to predict with some accuracy the characteristic frequency for each electrode of an implanted electrode array, so as to provide a reliable basis on which the allocation of frequency range to electrode mapping is made.
An article entitled "A cochlear frequency-position function for several species-29 years later" J. Acoust. Soc. Am. 87, 2592-2605, by Greenwood, D. D. (1990) describes the relationship between frequency and the site of maximal displacement of the basilar membrane expressed as a percentage of the total length of the organ of Corti, measured from the apex. The technique in this paper is not applicable to cochlear implants.
An article "Cellular pattern and nerve supply of the human organ of Corti" Bredberg, G. (1968).Acta Otolaryngol. (Stockh.) Suppl. 236, 1-138 describes temporal bone studies that establish a relationship between the percentage length along the organ of Corti and the angle in degrees about the modiolus relative to the basal end of the organ of Corti. This paper is not applicable to cochlear implants.
An article by Marsh, M. A., Xu, J., Blamey, P. J., Whitford, L. A., Xu, S. A., Silverman, J. M, and Clark, G. M. (1993). "Radiological evaluation of multiple-channel intracochlear implant insertion depth" Am. J. Otol. 14, 386-391 describes a method to document insertion depths of the electrode array from an X-ray. The paper identifies a difficulty in relating the angles that might be derived from the authors' method to the angles measured by Bredberg.
The above papers do not provide a clinically applicable method of accurately deriving the appropriate frequency ranges to be allocated to the electrode bands of a cochlear implant. Prior methods were based on the surgeon's reports, which the Marsh et al. paper states are inaccurate. Inaccurate prediction of the electrode/frequency correspondence leads to input frequencies mapped to the wrong sites in the cochlea. Such mis-mapping of electrodes may well result in a reduction in the comprehensibility and naturalness of sounds perceived by the implanted subject relative to that which would otherwise have been possible.